Ti64 alloy, having a nominal composition in weight percent of 6 percent aluminum, 4 percent vanadium, 0.2 percent oxygen, balance titanium and impurities, is one of the most widely used titanium-base alloys. The Ti64 alloy is an alpha-beta titanium alloy that may be heat treated to have a range of properties that are useful in aerospace applications. Ti64 alloy is used in both thin-section and thick-section applications, and heat treated according to the section thickness. In an example of interest, Ti64 alloy is used to make thick-section forged parts of aircraft gas turbine engines, such as compressor disks, fan disks, and engine mounts, which have at least some locations with a section thickness of greater than 2¼ inches. The present approach is concerned with such thick-section articles.
In the current best practice to achieve the optimal combination of strength and other properties, after forging the thick-section Ti64 articles are typically heat treated at a temperature of 1750° F., followed by an anneal heat treatment at 1300° F. The result is a 0.2 percent yield strength throughout the article of from about 120 ksi (“ksi” is an abbreviation for “thousands of pounds per square inch”) to about 140 ksi. This strength has been satisfactory for many thick-section applications.
To achieve higher yield strengths in the article, a more heavily alloyed, heavier forgeable alloy such as Ti17, having a nominal composition in weight percent of 5 percent aluminum, 4 percent molybdenum, 4 percent chromium, 2 percent tin, and 2 percent zirconium, is used. The Ti17 alloy uses a higher percentage of expensive alloying elements than does Ti64 alloy, with the result that a large, thick-section part made of Ti17 alloy is significantly more expensive than the same part made of Ti64 alloy.
There is a need for an improved approach to achieving excellent mechanical properties in forgeable titanium alloys. The present invention fulfills this need, and further provides related advantages.